Faculty Bibliography

Two aspects of the perception of extrapersonal space undergo systematic changes with variations in the pitch of the visual environment: (1) the physical elevation perceived to correspond to eye level (VPEL); and (2) the perception of the pitch of the visual environment (PVP). Thus, one might assume that both discriminations are controlled by a common mechanism utilizing visual information from the pitched surface. In fact this assumption has been made frequently, and - in different forms - underlies three substantial but very different historical streams in the literature. A quantitative theoretical development shows that two of these streams, although derived from very different viewpoints and appearing very different themselves (it is assumed that the basis for both PVP and VPEL is information about the pitch of the visual field in one, and information about the location of the subject's eye level within the visual field in the other), make identical predictions: each requires that the weighted sum of PVP and VPEL equal the magnitude of physical pitch and that the weighted sum of their first derivatives equal a constant. The third stream, which assumes that an internal representation of the visual field gives rise to both PVP and VPEL, requires that a weighted difference of PVP and VPEL be proportional to physical pitch and that the weighted difference of their derivatives equal a constant. In an experiment designed to examine the relation between VPEL and PVP, psychophysical measurements of VPEL and PVP were made on 20 subjects across a range of pitches from -30 degrees to +20 degrees. Contrary to the predictions from all three interpretations, we find no significant correlation between the two perceptual variables when the influence of pitch itself is removed, despite the fact that VPEL and PVP each increased systematically with increasing visual field pitch. The results not only rule out the specific predictions derived from all three historical streams, they also rule out any theoretical viewpoint that requires control of both perceptual responses by a single mechanism. The statistical independence between VPEL and PVP implies independence between the mechanisms that give rise to them. The correlation observed here and elsewhere between individual PVP and VPEL settings when the influence of the systematic variation of pitch is not eliminated is a consequence of the way in which variations in the two perceptions are generated experimentally, and not on an identity of the mechanisms mediating the generation of the two perceptual variables themselves.

A greater tendency to complete single-completion word stems (e.g. "BEY") to form previously read whole words (e.g. "BEYOND") was found when test stems were presented in the same letter case as their previously encoded words, compared with the different letter case, but only when stems were presented directly to the right hemisphere (i.e. in the left visual field) and not when they were presented directly to the left hemisphere (i.e. in the right visual field). This finding with single-completion stems was robust (i.e. observed for both lowercase and uppercase stems) when the initial encoding task was perceptually demanding, but it was test-case dependent (i.e. observed for uppercase but not lowercase stems) when the initial encoding task was not perceptually demanding. Results and theory help to explain why letter-case-specific priming in right-hemisphere test presentations is typically test-case dependent when priming is measured using perceptual identification at test, but is consistently robust when priming is measured using multiple-completion word stems (e.g."BEA") at test. Demands from both the stimuli and tasks affect the relative contributions of abstract and specific subsystems to the processing of visual forms.